How does trypan blue determine cell viability?

Trypan blue determines cell viability by using a hemocytometer and microscope. The process is this; after your sample of cells are suspended and treated with trypan blue, you then put sample onto a hemocytometer slide. Then, you must view the cells under a microscope under 10X power objective. Next, you must count the number of unstained cells in a single set 16 square grid. There are a total of 4 sets of these square grids, and you must count the unstained cells for each set. Then, you must calculate the average of the 4 sets and multiply that average by 10,000. You must then multiply by the dilution factor you have used, and that is going to be your cell viability in cells/ml.